1. Technical Field
The present invention relates to quantum computing and more particularly to systems and methods for calibrating and estimating performance of quantum computers.
2. Description of the Related Art
Building a quantum computer is a difficult task. Currently, one of the main problems faced by an experimenter in a laboratory is to verify that the device constructed performs as desired and is not hampered by factors such as environmental noise, cross-talk to other parts of a chip, low fidelity of a read-out mechanism, etc. While this task is currently in its infancy, it is expected that verification and testing of the parts comprising a quantum computer will be an important task for all future generations of such devices.
One type of noise that can affect the quantum computer is called energy relaxation, or simply relaxation. In current implementations based on superconducting flux qubits, this type of noise is the limiting factor of the life-time of the qubits. Understanding, characterizing, measuring, and eventually preventing it is an important goal on the way to a large scale quantum computer.
T. Ojanen, A. Niskanen, A. Abdumalikov, in “Global Relaxation Of Superconducting Qubits”, Phys. Rev. B, vol. 76, p. 100505(R), 2007, propose a method to determine whether relaxation is correlated or not. It has been proposed by Ojanen et al. to characterize such noise using a new experimental setup that will require sophisticated quantum operations to be applied to the qubits.
Ojanen et al. consider a particular entangling operation (i.e., a composite operation of 2 qubits) to perform a test for correlated or uncorrelated noise. However, such composite operations of 2 or more qubits can be difficult to apply. The methods described in Ojanen et al. have not been demonstrated in an experiment as they rely on architectures of superconducting qubit systems that are not available currently, however, it is expected that they will be available in the near future. The main limitation of Ojanen et al. is that the generation of highly entangled quantum states are needed, a difficult task that cannot be achieved with high reliability using current superconducting qubit technology and remains challenging even when making optimistic assumptions about how technology will improve.
In nuclear magnetic resonance (NMR) systems, methods have been experimentally demonstrated that permit determining some properties of quantum channels. For example, the problem of determining the average fidelity of a quantum channel has been considered. Intuitively, this is the problem of determining how much a quantum channel distorts a given input state when this state is compared to the output of the channel. The methods used for the solution also used twirling of the channels. However, these methods do not provide the type of noise characterization that is needed for many practical applications.